Method of manufacturing carbon fibres

ABSTRACT

A METHOD OF MANUFACTURING CARBON FIBERS, PARTICULARLY IN THE FORM OF A SHEET OR TAPE, IN WHICH A PLURALITY OF SHEETS OR TAPES OF POLYACRYLONITRILE FIBRES ARE FED THROUGH A FURNACE OR FURNACES IN WHICH AT LEAST THE MAJORITY OF THE CONSTITUENTS OTHER THAN CARBON ARE DRIVEN OFF, THE PLURALITY COMPRISING AT LEAST 10**5 FIBRES.

June 27, 1972 l. WHITNEY 3,673,035

METHOD OF MANUFACTURING CARBON FIBRES Filed April 15. 1969 BYMM fund- Horn-e y s United States Patent O METHOD F MANUFACTURING CARBON FIBRES Ian Whitney, Wrksworth, England, assignor to Rolls- Royce Limited, Derby, England Filed Apr. 15, 1969, Ser. No. 816,202 Claims priority, application Great Britain, Apr.l 19, 1968, y 18,617/ 68 Int. Cl. C011) 31/07, 47/32; B32b 31/12 U.S. Cl. 156-235 6 Claims ABSTRACT oF THE DISCLOSURE A method of manufacturing carbon fibres, particularly in the form of a-sheet ortape, in which a plurality of sheets or tapes of polyacrylonitrile libres are fed through a furnace or furnaces in which at least the majority of the constituents other than carbon are driven off, the plurality comprising at least 105 fibres.

This invention relates to a method of manufacturing carbon fibres and provides a method by which tapes or sheets may be manufactured in a continuous, integrated process.

According to the present invention a method of manufacturing carbon fibre comprises continuously feeding a plurality of tapes or sheets of fibre of polyacrylonitrile or a copolymer thereof in parallel through a furnace or furnaces wherein the majority of the constituents other than carbon are driven off, said plurality comprising at least 105 fibres.

The method may also comprise the step of continuously bringing together a plurality of fibres of polyacrylontrile or a copolymer thereof to form each of said tapes.

Said tapes may be fed initially through a pre-oxidising furnace in Which a temperature of between 150 C. and 300 C. is maintained in an atmosphere of an oxygencontaining gas.

After emerging from said pre-oxidising furnace said tapes may be fed to a carbonising furnace in which a temperature of between 500 C. and 1200 C. is maintained in an inert atmosphere.

The carbon fibre tapes are preferably subsequently resin impregnated.

Said resin impregnation is preferably effected by the transfer of resin from coated release paper to the tape by a combination of heat and pressure.

Said coated tapes produced are preferably subsequently cut traversely to form sheets.

The invention also comprises the carbon fibre made by the method set out in any of the above statements.

The invention will now be particularly described merely by way of example with reference to the accompanying drawing which is a flow diagram of apparatus for carrying out the method of the present invention.

In the diagram, indicates a creel unit in which a plurality of creels 11 hold tows of polyacrylonitrile fibres of 11/2 denier. In this creel unit there is embodied a tape or sheetforming and retaining unit (not shown) such for instance as is described in our co-pending application Ser. No. 792,646 (now abandoned). The creel unit produces tapes or sheets held together by a weft thread introduced at intervals into the warp fibres, each sheet being of approximately 18 inches width and each containing 50 tows, each tow comprising 104 fibres. The eight tapes are maintained with a separation of some three inches between tapes, the tapes lying one above the other, and are fed into a roller unit generally indicated at 12. The roller drive unit 12 comprises eight separate sets of four rollers, one of which is'indicatedA at 13. These sets of rollers are mounted in parallel so that each roller set receives a single tape lCe which then passes round the four driven rollers which comprise the set and then passes out from the drive unit horizontally aligned with its entry.

On passing from the drive unit 12 the eight tapes are brought closer together as shown at 14 and pass through closely fitting orifices at 15 into a pre-oxidisation furnace 16. The atmosphere inside the furnace is maintained by a blower system and heater units (not shown) so that the ambient atmosphere within the furnace comprises air heated to a temperature of some 220 C. A blower system 17 is provided to effect a controlled flow of air in the furnace.

Within the furnace 16 are a plurality of sets of rollers indicated by 18. The eight tapes pass over these rollers which cause them to move over a castellated path within the furnace, in this way achieving a long path length (40 feet) within a relatively compact furnace; the tapes are arranged to take some seven hours to pass through the furnace.

While within the furnace 16 the fibres are therefore pre-oxidised, that is some degree of modification takes l place between individual molecules to stabilise their structure.

On emerging from the furnace through closely fitting orifices at 19 the fibres, still in their tape form, pass into a second roller drive unit 20, which is identical to that shown at 12. By arranging the drive units 12 and 20 at either end of the furnace 16 the fibre can be arranged to be held` under tension while it is being pre-oxidised. This prevents .the shrinkage which would otherwise take place during this stage of the process. On leaving the drive unit 20 the tapes are brought together to form a single bunch and pass into a carbonising furnace 21 by way of a roller seal unit 22. This seal unit makes use of a pair of roller seals as described in our copending application Ser. No. 771,466 (now abandoned) and prevents any considerable leakage from the furnace 21. The fibre sheets are compacted at this stage to reduce the power required and to simplify the system; at this stage it is unnecessary to maintain the spacing which is required in the previous stages to allow purging of reaction products and dissipation of heat.

In the furnace 21 a nitrogen atmosphere is maintained, a duct 23 being provided for the influx of nitrogen while a duct 24 is provided for the efflux of nitrogen plus waste gases. Inside the furnace 21 the tapes are suspended above a floor (not shown). Within the furnace the temperature is arranged to rise steadily from room temperature adjacent to seal unit 22 up to approximately 1000 C. at a point 25. The dimensions of the furnace are so chosen that at the point 25 the majority of the pyrolysis of the fibres has taken place; this being so there is no necessity to arrange for any additional purging of reaction products in the subsequent treatment.

The compacted fibres next pass through a higher temperature portion 26 of the furnace. It will be noted that the sectional area of this part of the furnace is reduced by wall assemblies 27 which comprise heating units; this reduction in area helps to reduce the power required to provide the higher temperature heating. In the section 26 the fibres are further heated to a temperature in the region of 1600 C.

The compacted fibres finally emerge from the furnace 21 by way of a second seal unit 28 which again uses roller seals having an evacuated space therebetween to ensure complete removal of toxic reaction products.

On leaving the seal unit 28 the fibres pass into a further furnace 29. Although for convenience of drawing this furnace is shown as being separate from the furnace 21, it is in fact connected directly to the seal 28 and the entire apparatus forms a linear lay-out. Thus the seal 28 in fact divides the atmosphere within the furnace 21 from that in the furnace 29. Within the furnace 29 an atmosphere of argon is maintained and once again an input pipev 30 supplies argon while an output pipe 31 allows egress of argon and any products of reaction. Within the furnace 29 the libre is heated, still in its compacted form, to a temperature in this case of 2500 C., but which may be in the range of 1500 C. upwards. In this stage the structure of the carbon fibre is made more pseudo-graphitic. This stage can in fact be omitted completely and it is possible `to produce useful carbon libre with la carbonising furnace which merely heats the libre up to a temperature of 1000 C. to 1200 C.

The libre exits from the furnace 29 after some 1/2 hour residence time by way of a seal unit 32 which is given similar to the 4unit 28 and prevents large escape of gases from the furnace 29. Immediately on leaving the seal unit 32 the compacted libres pass between a pair of driven rollers 33 and after passing through these rollers the compacted libres are again split up into their eight discrete tapes into a resin impregnation unit generally indicated at 34. The resin impregnation unit 34 is of the type described and claimed in our co-pending application Ser. No. 887,864 and uses coated paper as a transferring agent for an epoxy Novolac resin. For simplicity, the course of a single tape will be described; the remaining tapes follow an exactly similar course.

Considering the upper tape, first, on entering the unit 34 it passes between a pair of reels 35 and 36 of resin coated release paper. The release paper is itself impregnated with a release agent which alfords the resin no permanent adhesion to the paper. In order to make the coated paper easier to handle, the reels 35 and 36 are mounted within a chamber 37 which is refrigerated so that the resin is maintained non-tacky.

The paper from the reels 35 and 36 is fed on to either side of the libre tape so that the coatedside of the paper is against the libre. The sandwich thus formed passes from the chamber 37 into a second chamber 38 and between a pair of heated rollers 39. On passing between these rollers, the combined effect of the heat and pressure evolved causes the resin to transfer from the release paper to the libre tape, thus impregnating the tape with resin.

On leaving the rollers 39 the sheets come together in a stack, and pass through rollers 40. The stacked sheets then pass through a conventional guillotine device 41 and are cut into sheets which are transported by a conveyor device 42 to be stacked at 43. They may alternatively be rolled on eight drums.

It will be noted that resin impregnation could be carried out by alternative methods such as immersion or spraying.

It will be appreciated that the present invention, by using a multiplicity of fibres passing at a relatively slow speed through a series of furnaces enables a good output to be maintained with considerable advantages in ease of processing etc. It will be particularly noticed that the method of the present invention produces libres in sheet form; this form is generally highly convenient for the laying up of articles from the libres.

What We claim is:

1. In a continuous method for manufacturing large numbers of carbon fibres from libres of polyacrylonitrile,

Y 4 f or a copolymer thereof, the improvement comprising the stepsofv forming a lirst plurality of tapes or sheets from large numbers of individual continuous length libres formed from polyacrylonitrile or copolymer thereof, said plurality of said tapes or sheets comprising a total of at least libres, feeding said lirst plurality of said tapes or sheets, spaced apart and one above the other in generally parallel relationship, through a lirst furnace, heat treating said plurality of tapes or sheets while in said lirst furnace in an oxygen-containing atmosphere to a temperature between C. and 300 C.,

withdrawing said first plurality of tapes or sheets from said lirst furnace and compacting the tapes after exit from the first furnace to form a single tape,

feeding said compactedtape through a second furnace,

heat treating the fibres while in said second furnace at a temperature of between 500 C. and 1200 C. in an inert atmosphere to drive olf a majority of the constituents, other than carbon, of the libres, and separating the large number of individual libres making up said single tape into a second plurality of tapes or sheets after heat treatment in said second furnace.

2. The method of claim 1 and including a step of maintaining a tension on said tapes or sheets while being treated in said first furnace.

3. A method as claimed in claim 1 and in which said single tapes, after emerging from the second furnace is fed` ythrough a graphitizing furnace in which a temperature of between 1500 C. and 3000 C. is maintained in inert atmosphere.

4. A method as claimed in claim 1 and comprising a resin impregnation step in which said second plurality of tapesl of carbon libre are impregnated with a resin materia 5. A method as claimed in claim 4 and in which said resin impregnation is elected by the transfer of resin from coated release paper to the tape by a combination of heat and pressure.V

6. A method as claimed in claim 5 and in which the coated tapes produced are subsequently cut transversely to form sheets.

References Cited UNITED STATES PATENTSl 2,485,725 10/ 1949 Francis, Jr. 156-235 X 2,868,269 1/1959 Letteron 156-230 X .3,174,895 3/ 1965 Gibson et a'l. 23-209.2 UX 3,285,696 11/1966 Tsunoda 23,-209.1 F 3,412,062 11/ 1968 Johnson 23-209.1 2,875,501 3/1959 Gravley 156-155 3,129,129 4/1964l Schrode 156-161 3,552,923 A1/ 1971 Carpenter et al. Z13-209.1 F

CARL D. QUARFORTH, Primary Examiner R. S. GAITHER, Assistant Examiner U.S. Cl. X.R. 

